Labour's flagship reforms to protect women from domestic violence have failed to increase criminal convictions, a government study has found.

Tough laws unveiled in 2003 were hailed by ministers as the "biggest overhaul of legislation on domestic violence in 30 years".

But the Government's own early evaluation of its success has concluded that the impact of the Domestic Violence, Crime and Victims Act (DVCV) 2004 is "limited" and may have even deterred some women from pursuing their complaints through the criminal justice system.

Under the new legislation, the police were given the power to make common assault (where the victim only fears violence) an arrestable offence and the courts were given the power to jail defendants for five years for breaching a non-molestation order.

But in Croydon and South Tyneside, the two areas studied by the researchers, convictions for domestic violence have actually fallen to a three-year low. Last year in South Tyneside, there were 247 reports of domestic violence of which only eight ended in a conviction. In Croydon, where the Government has set up a specialist domestic violence court, only seven of the 378 allegations made last year had resulted in a conviction by the time the researchers had compiled their report.

Of equal concern was the marked fall across the country in the number of requests for molestation orders after the five-year prison sentence was introduced last year. The report, commissioned by the Ministry of Justice, said: "There was a reduction in the number of applications and orders following July 2007 when the new measure was implemented but this reduction was more pronounced for orders than it was for applications. The decreases are larger than in previous years.

"It must also be remembered that the DVCV Act actually extended the availability of non-molestation and occupation orders and hence we would expect to see an increase in orders granted and not a decrease overall."

The picture was similar when the researchers looked at domestic violence victims who did not live with their partner but now benefited from the power. Lawyers, refuge centre workers and others told the authors they thought one reason for the fall in the orders was the potentially draconian five-year prison sentence for breaching an order.

The report, which conceded it would take longer for the measures to become better known, concluded: "Those engaged with the court process felt there had been a reduction in applications for non-molestation orders and orders granted since July 2007 either due to both a reduction in the availability of legal aid and the criminalisation of breaches, or because victims were concerned about potential imprisonment that follows a breach."

The area of greatest success was in relation to common assault being made an arrestable offence. Here there were "tentative" signs that this power of arrest was being put to greater use in domestic violence cases. Women's groups said the report was a warning to the Government not to rely on new criminal punishments alone to tackle domestic violence. Heather Harvey, campaign manager of Amnesty International UK's Stop Violence Against Women, said: "Although the Government has made some important and effective strides in tackling domestic violence, it is undermining its own efforts because the measures are isolated and unsupported, as this study suggests ... However strong one initiative may be, it will have a limited impact if it's not inextricably linked across all areas of policy, and so women at risk of violence will continue to fall through the gaps."

Bridget Prentice, the Justice minister, said the Government had made great progress in tackling domestic violence. Speaking after the report was published, she said: "Government figures show the number of domestic violence cases being reported has increased. The success rate on prosecuting domestic violence continues to improve.

"In 2007, both family and criminal jurisdictions of court helped 22,975 more victims than we did in 2004. This is a significant improvement against the background of an increasing volume of prosecutions. This is a testament to the growing confidence victims have that they will be listened to and their reports effectively investigated coupled with the specialist and practical support victims receive."

But Nicola Sharp, policy director of Refuge, said: "Because the police and the CPS have targets to meet, what we also find is that when arrests are made, perpetrators are only charged if the CPS considers it a case that is likely to be successfully prosecuted and this leads to offences often being downgraded or dropped altogether."

'My husband threw me on the floor and raped me' - Joanna Tidings, victim

One night Joanna Tidings was soaking in the bath when her husband, David, came back unexpectedly. He walked into the bathroom, smiling. He had been moody recently, but she thought he was pleased to see her. Instead, he dragged her out of the bath by her hair, banged her face on the sink, threw her on the floor and raped her.

At university, David had seemed so charming and sexy. Only once, before they got engaged, had he slapped her.

But after Joanna and David got married, all her friends gradually disappeared. The day after she was raped, Joanna went to the police.

Eventually she decided not to press charges. However, she kept wondering whether she could have done anything differently to stop the attack, and she also felt betrayed.

Plagued by these feelings, she went to her local rape crisis centre where she is being helped to deal with her abusive relationship.

The names in this article have been changed